Study On Soil Dissolved Organic Carbon Loss And Reduction Under Different Land Use/Cover Types At The Confluence Of Hongze Lake And Huai River

Land use/cover along rivers is an important ecological sensitive area,and its contribution to water quality change can be shown as ecological buffer zone that blocks the entry of source factors and pollutants,but it may also input its own substances into rivers as pollution sources.It is necessary to scientifically understand the rule of soil loss under coastal land use/cover to prevent and control river pollution.Hongze Lake is the fourth largest fresh water lake in China.It carries more than 70%of the water volume of the Huaihe River,forming a typical wetland in the eastern lake entrance area where rivers,streams,creek,ditch,bay and ponds cross.For a long time,with the accelerated influence of natural succession and human activities,wetland in the intersection area of river and lake has not only natural vegetation such as silt beach and reed,but also man-made land use such as plantation and farmland,which has become a unique ecological system with both ecological and production functions in the region.With the development of national ecological civilization construction and"two-carbon"strategy in recent years,wetland carbon sink research has attracted much attention.In the past,important progress has been made in the researches on organic carbon in coastal riverfront soil.However,the researches on the traceability of organic carbon loss,river sediment and loss resistance control based on coastal land use/cover are still insufficient,especially in the Hongze Lake area.In this paper,at the confluence of Hongze Lake and Huai River is taken as the prototype area.Five types of different land use/cover types,including natural cover types of lake grass flat and reed flat,willow and poplar plantation,and farmland,are selected in the typical reach of the river.Samples of soil,river sediment and water are collected.Dissolved organic carbon（DOC）and its spectral characteristics,soil organic carbon（SOC）,nitrogen（N）,phosphorus（P）and other indexes were determined,and DOC fluorescence index（FI）,autogenic index（BIX）,humification index（HIX）were calculated to study DOC leaching characteristics and influencing factors.The contribution of different land use/cover types to SOC of river channel sediments was revealed by usingδ¹³C andδ¹⁵N stable isotope ratios combined with fingerprint identification method.The inhibition effect of DOC loss was investigated by the addition of biochar（biochar to soil mass ratio:0 and 3%）.By analyzing the correlation between soil and sediment of coastal land use/cover type and its source,this paper is helpful to further understand the ecological sensitivity and rationality of land use/cover type along river banks,provide scientific basis for regional wetland ecological environment protection,and help ecological civilization and the realization of"two-carbon"strategy.The main research results and conclusions of this paper are as follows:（1）Both poplar and willow plantations had the highest leaching amount and loss rate of DOC,lake-grass beach and reed beach were at a low level,and farmland was in the middle.At the same time,the simultaneous loss of N hormone per unit DOC was the highest in farmland and lake-grass beach,and the lowest in willow forest and poplar forest.The loss of P was the highest in lake-grass beach and reed beach,and the lowest in willow forest and poplar forest.This indicates that although the land use of nearshore plantation may lead to higher soil DOC input into rivers,it effectively reduces the simultaneous loss of N and P.（2）Under different land use and cover patterns,the aromatic and hydrophobic properties of poplar forest and willow forest were the highest,and their molecular weight was small,while the aromatic and hydrophobic properties of lake grass beach,reed beach and farmland were weak,and their molecular weight was large.Further analysis showed that the loss of DOC was positively correlated with its aromaticity and hydrophobicity（P<0.01）,positively correlated with soil EC and SOC,DOC and TN contents,and negatively correlated with soil C/N.It can be seen that reducing the aromatics and hydrophobicity of DOC or increasing the utilization rate of N is expected to reduce the loss of DOC.（3）The aromaticity of DOC in river sediment was similar to reed beach and farmland,and the molecular weight of DOC in poplar forest soil was significantly higher than that in sediment.The results of DOC three-dimensional fluorescence spectra showed that C1:C2:C3:C4（1:0:87:0.45:0.70）was highly similar to that of lake grass flat and reed flat（1:0:87:0.44:0.81）.There was a significant difference between forest land and farmland（1:0.75-0.84:0.38-0.53:0.23-0.50）.The variation of FI,BIX and HIX values showed that there was no significant difference between channel sediment and lake-grass beach,reed beach and farmland,but significant difference between channel sediment and forest land.Therefore,it is inferred that C in channel sediments has a high homology with reed beach,lake-grass beach and farmland soil,which is probably mainly from the contributions of the latter three.（4）Theδ¹³C andδ¹⁵N values of river channel sediments were similar to those of lake-grass beach and farmland soils,but significantly higher than those of willow and poplar forests.The bivariate model ofδ¹³C,δ¹⁵N,and C/N indicates that the main sources of SOC in river channel sediments are lake-grass beach,reed beach,and farmland soil.The SIAR model quantified the SOC contribution ratio of river sediment,and the results showed that reed beach contributed the most（41.8%）,followed by lake-grass beach（23.8%）and farmland（21.3%）,poplar forest（7.2%）and willow forest（5.9%）contributed less.（5）After the addition of biochar,the DOC loss of reed beach decreased by 48.4%,willow forest by 12.3%and poplar forest by 20.7%.The steady loss rate of soil DOC in farmland and willow forest decreased by 39.4%and 37.1%,respectively.The total soil DOC loss in reed beach,willow forest and poplar forest decreased by 22.7%,11.0%and 4.68%,respectively.The change characteristics of DOC loss rate and total DOC loss decreased significantly,but there was no significant difference in other land use/cover modes,but the TN loss of poplar forest,willow forest and farmland decreased significantly,by 38.2%,26.0%and 22.2%,respectively.The total soil TP loss of lake grass beach,farmland and poplar forest increased.At the same time,the simultaneous N loss of poplar forest,farmland and willow forest decreased significantly per unit DOC loss after biochar addition,but the TP loss of lake-grass beach and farmland increased significantly.（6）The addition of biochar did not change the significant positive correlation between DOC loss and its aromatics and hydrophobicity,but the positive correlation between TN loss rate and the aromatic and hydrophobic components of DOC（without the addition of biochar）changed to a negative correlation（with the addition of biochar）.In conclusion,the organic carbon loss of natural cover of lake-grass beach and reed beach was the least,but the organic carbon traceable analysis of river sediment showed that these two types of cover contributed the most to the DOC loss（65.6%）.This indicates that although the leaching amount and loss rate of DOC in poplar and willow plantations are high,it may not enter the river channel in large quantities.Because of the aromatic and hydrophobic properties of DOC in poplar and willow forests and their low molecular weight,it is likely that DOC in poplar and willow forests will vertically enter the soil and promote the storage of organic carbon.Therefore,if it is further considered that the loss of unit DOC in the lower lake will result in simultaneous loss of N and P in the grass flat and reed flat,while the N and P of willow forest and poplar forest remain at a low level,from the perspective of water quality improvement at the confluence of Hongze Lake and Huai River,the optimal land use/cover method on both sides of the river is poplar and willow ecological forest.This proposal is also consistent with the current status of the region’s dual functions of ecology and production.In addition,it is recommended to add biochar to the soil of poplar and willow ecological forests to further control N loss.